1. Field of the Disclosure
This invention pertains to a method for the preparation of printing forms, and particularly for the preparation of composite printing forms for use in relief printing.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials including corrugated carton boxes, cardboard boxes, continuous web of paper, and continuous web of plastic films. Flexographic printing plates are a form of relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions that typically include an elastomeric binder, at least one monomer, and a photoinitiator, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element.
Corrugated boxes and other relatively large objects that are printed using relief image printing plates often bear actual printing on only a small portion of their total surface area. That is, only relatively small areas of the corrugated substrate need to be printed compared to the overall size of the component being formed from the corrugated substrate. Since only portion of the surface of the substrate needs to be printed only small portion/s of the single plate will actually be used for ink transfer to create the printed image/s, and large portions of the printing plate will not be used, i.e., have no image. Those skilled in the art often print relatively large objects and corrugated board substrates with composite printing plates or composite printing forms. Composite printing forms are prepared by creating one or more relatively small (compared to the overall size of the substrate being printed) relief image printing plates or printing portions, on a carrier at location/s that correspond to image/s that is to be printed on the corrugated substrate.
One method to prepare a composite printing form, which sometimes referred to as in-position workflow, utilizes a single printing plate to create the relatively small relief image portions that remain intact on the support of the plate. In some instances of in-position workflow, the large unimaged portions, i.e., unused portions, of the single printing plate are removed by cutting through the photopolymerizable layer surrounding the printing portion/s, and stripping the unused portions of the layer from the support prior to washout. Examples of in-position workflow are disclosed in U.S. Pat. Nos. 5,292,617; 5,460,920; and 5,484,684. However, stripping or removal of the unused portions of the photopolymerizable layer from the support can sometimes be difficult or cumbersome and may disrupt the small portions remaining on the support. In particular, manual stripping of the unused portions of a very thick photopolymerizable layer, that is a photopolymerizable layer having a thickness of about 150 to 270 mils (0.381 to 0.686 cm), from the support can be cumbersome and problematic.
In other instances of in-position workflow, the unused portion/s, i.e., unimaged portions, of the single relief printing plate are removed by washout processing at the same time as the portion/s of the relief image is being formed. One example of in-position workflow with washout is disclosed in U.S. Pat. No. 4,927,723. However, removal of the unused portion places extreme stress on the washout apparatus and on the relief image plate. Excessively long washout time is needed to try to remove the unused portions of the plate, which can cause increased swelling of the plate and/or increased throughput of solvent and output of volatile organic components. It is often very difficult to washout the unused portions of the photopolymerizable layer completely down to the substrate without damaging the raised relief portions of the one or more printing areas. Oftentimes some small section/s or a relatively thin layer of the unused portion/s remains on the substrate after washout, which because of its size and the characteristics of the material, is too difficult to completely strip or remove from the substrate. The composite printing form composed of the relief image portions remaining on the support may be mounted directly on the printing cylinder, or more typically may be mounted onto a carrier sheet which is then mounted on the printing cylinder.
Another way to print corrugated board substrates and other large objects is to prepare a composite printing form having one or more relatively small image portions, or printing areas, individually mounted onto a carrier at locations that correspond to the print image areas on the substrate. The relief image on the relatively small image portions, sometimes referred to as slugs, may be prepared individually or, collectively from a relief printing plate and then separated into the individual portions. Selective mounting of the small relief image portions on the carrier requires careful adhesion of the individual portions to the carrier while assuring accurate registration, in order to print high quality images and multi-color reproduction.
Other methods of preparing composite printing plates have been suggested, such as in U.S. Pat. No. 5,846,691; U.S. Pat. No. 6,312,871; U.S. Pat. No. 6,312,872; U.S. Pat. No. 6,399,281; and U.S. Pat. No. 6,472,121, which do not require precise registration of constituent photocurable elements. Generally the methods involve disposing at least one photocurable element upon a surface of a carrier in approximate register and then transferring a computer generated negative to a surface of the elements. The methods include transferring registration information of any visually perceptible modification of the printing element, i.e., carrier, that is intended to reflect positioning of the photocurable elements. The negative or mask may be generated by jetting ink onto the surface of the element or by exposing with laser radiation to selectively remove a radiation opaque layer from the surface of the element.
In those instances in which the relatively small image portions or slugs are selectively mounted on the carrier at locations that correspond to the print image areas, oftentimes a problem occurs during printing in that one or more of the slugs lift or loosen from the carrier. Printing inks generally include one or more solvents that can attack an adhesive or tape used to mount the slugs onto the carrier. The ink solvent/s changes the adhesion of the slug along its perimeter, weakening the attachment of the slug to the carrier causing the slug to slip, shift, or completely lift off of the carrier.
There remains a need in the art for an alternative process for preparing a composite printing form from constituent image portions that avoids the problems and/or overcomes the difficulties of prior methods for preparing composite printing forms. In particular, there remains a need for an alternative, easy to use, process for preparing the composite printing form that includes relatively small image portions which are constituent relief image printing plates for the composite printing form.